The following is a very quick summary. See the Sweeping Wind rune effects page for a more thorough description of rune effects, or any of the individual rune pages for numbers, screenshots, blue quotes, and much more.
Surround yourself in a vortex that continuously deals 30% weapon damage to all enemies within 10 yards. The vortex lasts 6 seconds and is refreshed each time you strike an enemy with a melee attack. Landing a Critical Hit has a chance to increase the vortex effect up to 3 stacks for a total of 90% weapon damage.
While your vortex is at the maximum stack count, Critical Hits have a chance to spawn a lightning tornado that periodically electrocutes nearby enemies for 23% weapon damage as Lightning. Each spawned lightning tornado lasts 3 seconds.
During the beta, all the skills previously known as combos were simplified down so much that only three of them (Fists of Thunder, Deadly Reach, and Crippling Wave) still contain any semblance of combo functionality. These three skills now provide simply an additional effect on every third strike, while the others (Way of the Hundred Fists, Exploding Palm, and Sweeping Wind) now have a single function on every strike. Combos previously consisted of: the normal spell attack on the first strike; a stronger attack on the second; and on the third, an even stronger attack coupled with an additional effect.
Skill tiers were removed shortly before the beta started in September 2011, and Sweeping Wind became a level 17 skill. The skill system was once again overhauled for the Beta Patch 13 in February 2012, and Sweeping Wind was classified as a Techniques skill and moved to level 22.
For Beta Patch 14, it was moved to level 21. It was also significantly changed in function: previously, it was a Spirit Generating combo skill that struck a single target on every hit, and every third hit created the vortex effect.